The field of display device is very important for the information and communication industry. Recently, in accordance with a speed-up in the development of information and communication technology, more highly-advanced efficiency has been asked for in this field. Display can be divided into luminescent type and non-luminescent type. The luminescent type of display comprises Cathode Ray Tube (CRT), Electroluminescence Display (ELD), Light Emitting Diode (LED), Plasma Display Panel (PDP), etc. The non-luminescent type of display comprises Liquid Crystal Display (LCD), etc.
These luminescent and non-luminescent types of displays have such basic properties as operation voltage, consumption power, brightness, contrast, response rate, life time, etc. However, LCD, which has been widely used up to now, has some problems in response rate, contrast, and sight dependency among the above basic properties. On the contrary, LED-using display can solve the above LCD problems, and also has many other advantages such as fast response speed, no need of back light due to self-emission, and excellent brightness. Thus, it is anticipated that LED-using display becomes the next-generation display device.
However, LED is mainly used with a crystal form of inorganic material, and so is hard to be applied to a large size of electroluminescent device. In addition, the electroluminescent device using inorganic material is very expensive and needs more than 200 V of operation voltage. However, Eastman Kodak reported in 1987 that the company manufactured a device made of a material having π-conjugate structure such as alumina quinine. The study for electroluminescent device using organic material has been more active thereafter.
The electroluminescence device (EL device, below) can be divided into inorganic EL device and organic EL device, depending on what material is used to form the emission layer (emitter layer).
The organic EL device, a self-emitting type of device that electrically excites fluorescent organic compound, is superior to the inorganic EL device in brightness, operation voltage, and response rate, and also can emit multi-colors.
In addition, the organic EL device is a luminescent device to emit in low voltage current, and has superior properties such as enhanced brightness, high speed of response, wide view angle, plane luminescence, slim type, and multi-color luminescence.
Thus, the organic EL device is expected to be applicable to a full-color flat panel display due to such superior properties that cannot be found in other displays.
C. W. Tang et al. reported the first practical device performance of the organic EL device in Applied Physics Letters, vol. 51 (12) pp 913-915 (1987). They developed a structure laminated with a thin film (a hole transport layer) obtained from diamine analogues and a thin film (an electron transport layer) obtained from tris(8-quinolinolate)aluminum (Alq3, below) as organic layer. The laminated structure can lower the injection barrier of electron and hole from both electrodes to the organic layer, and also can enhance the re-combination probability of electron and hole from the inner organic layer.
Later, C. Adachi et al. developed an organic EL device having an organic luminescent layer with three-laminated structure of hole transport layer, emission layer, and electron transport layer [Japanese Journal of Applied Physics, vol. 27 (2), pp L269-L271 (1988)], and two-laminated structure of hole transportable emission layer and electron transport layer [Applied Physics Letter, vol. 55 (15), pp 1489-1491 (1989)], and showed that the optimization of device property can be achieved by constructing a multi-layer structure suitable for materials and combination thereof.
The general organic EL comprises a first electrode (anode), a second electrode (cathode), and organic luminescent media. The organic luminescent media have at least two separate organic luminescent layers, i.e. one layer to inject and transport electron, and the other layer to inject and transport hole into the device. In addition, another multi-layer of thin organic film may be included. The above layers to inject and transport electron and hole each can be divided into an electron injection layer, an electron transport layer, a hole injection layer, and a hole transport layer. In addition, the organic luminescent media may be constructed with further including an emission layer besides the above layers.
The simple structure of organic EL device comprises a first electrode/an electron transport layer, and an emission layer/a second electrode. Also, the structure of organic EL device can be separated into a first electrode/a hole injection layer/a hole transport layer/an emission layer/an electron transport layer/an electron injection layer/a second electrode.
The operation principle of the organic EL device having the above structure is as follows.
If voltage is applied to the above anode and cathode, the hole injected from the anode is transferred to the emission layer via the hole transport layer. Meanwhile, the electron is injected from the cathode to the emission layer via the electron transport layer. The hole and electron are re-combined in the emission layer to form exiton. The exiton is changed from the excitation state to the basic state, by which the fluorescent molecule of the emission layer becomes luminescent to form images.
At present, the material conventionally used for the hole transport layer is triphenylamine analogues. In addition, organic metal complex compounds or heterocyclic compounds are used for the electron transport layer. Organic compounds or organic metal complex compounds are solely used for the emission layer or as host of the emission layer. When organic compounds or organic metal complex compounds are used as host of the emission layer, organic luminescent materials or a metal complex type of organic luminescent materials are used as dopant, thereby controlling the color of luminescence.
The maximum quantum efficiency of luminescent materials used in an organic EL device is about 5% by theoretical calculation. If such low quantum efficiency can be enhanced, the life time of the device may be increased. Generally, fluorescence is light emitted when the molecule is fallen from the monoplet excitation state to the basic state. On the other hand, phosphorescence is light emitted when the molecule is fallen from the triplet excitation state to the basic state. In case of fluorescence, the maximum efficiency emitted from the basic state of molecule is about 25%, and in case of phosphorescence, about 75%. That is, phosphorescence has high luminescence efficiency than fluorescence, by which it is possible to extend the life of the device. Particularly, to put the full-color display into practice, it has been urgently needed to develop a material having high purity red luminescence. A present study concerns platinum metal complex organic compounds which are phosphorescence materials, as red luminescence materials having high purity and efficiency for the organic EL device (U.S. Pat. No. 6,310,360).
Metal complex organic compounds to constitute the emission layer have a different luminescent color in accordance with the molecular structure of ligand. In this case, the emission layer comprises only platinum metal complex organic compounds of phosphorescence materials, or includes platinum metal complex organic compounds of phosphorescence materials as dopant. However, phosphorescence materials having practical luminescence efficiency have not been developed yet.
In view of the above, the present inventors have conducted extensive studies to develop novel phenyl pyridine-iridium metal complex compounds of formula (1) having practical luminescence efficiency, and completed the present invention.